You just need to pick a fitting trigger -- I picked File Events for this example:

Condition: File Events

Task: WiFi Tether

So with that set up (specifying the parameters of course), you should be able to trigger your WiFi hotspot (aka "WiFi Tether", as the Tasker manual names it) by placing a file -- and trigger it off again by removing the file. Did not try that myself -- but that's how I understand it :)